Polyvinyl alcohol (PVOH) has long been used as a protective colloid in the manufacture of polymer emulsions to stabilize such emulsions. The literature teaches that, for example, 2% to 10% of PVOH, based on the weight of the monomers being polymerized, may be used as an effective stabilizer in emulsion polymerization. The surfactancy and the effectiveness as a protective colloid both increase as the residual acetate content increases. Such PVOH stabilized emulsions contain almost exclusively polyvinyl acetate, and if other monomers are used, surfactants may be required to keep the latex polymer particles suspended and the emulsion stabilized.
The use of PVOH as a stabilization agent in vinyl acetate homo- and co-polymerization is widely known and has been illustrated in a number of patents. Recently, three patents or applications have been published which utilize polyvinyl alcohol in some way to prepare stabilized butadiene and styrene-butadiene copolymer emulsions. The first publication (Japanese Patent No. 61 79,705) teaches a two-step process to produce a PVOH-stabilized polymer. The first step involves making thiol-modified polyvinyl alcohol, which is produced through a co-polymerization of vinyl acetate and thiolactic acid or thioacetic acid and subsequent hydrolysis. In a second step, the modified PVOH polymer is used to stabilize a styrene-butadiene emulsion. The process used in the present invention does not use thiol groups.
The second publication (EP-A 516,360) utilizes a co-solvent such as methanol during the emulsion polymerization of butadiene to facilitate the interaction and stabilization of the latex by the polyvinyl alcohol. It is believed that the presence of the stabilizing solvent fosters the grafting of the polybutadiene onto the polyvinyl alcohol during the emulsion polymerization. In a subsequent step, the methanol solvent is removed through a stripping process to yield the final polybutadiene-polyvinyl alcohol latex. The present invention does not involve the use of a solvent but nevertheless yields stable water-based latex emulsions.
The third publication (U.S. Pat. No. 5,830,934) teaches to utilize a functionalized silane component that is incorporated into a growing emulsion polymer. The use of the silane facilitates the adsorption of a protective colloid such as polyvinyl alcohol onto a butadiene-containing emulsion copolymer to provide stabilization to the polymer. The copolymers of the present invention do not require a functional silane as a necessary component which is incorporated into the polymer to provide polymer stability.
EP 44 995, issued to Cassella A G, teaches the preparation of graft copolymer dispersions wherein vinyl compounds such as N-vinyl-N-methylacetamide are grafted onto a high molecular weight PVOH backbone. In SU 798212 (Anastasovski et al.), the preparation and application of acrylonitrile/methyl methacrylate/PVOH grafted latexes is described.
JP No. 50-34630 (Ogura et al.) describes the synthesis of modified PVOH/acrylic latexes. In this invention, the PVOH is first partially urethanated prior to using it in an emulsion polymerization with a (meth)acrylic ester monomer.